Hard disk drives store information on magnetic disks. Typically, the information is stored on concentric tracks of the disk that are divided into servo sectors and data sectors. Information is written to or read from the disk by a transducer or head, mounted on an actuator arm that positions the transducer over the disk in a predetermined location. Accordingly, the movement of the actuator arm allows the transducer to access the different tracks of the disk. The disk is rotated by a spindle motor at a high speed, allowing the transducer to access different sectors within each track of the disk.
A servo control system is utilized to position the actuator arm. The servo control system, which includes a voice coil motor associated with an actuator assembly, performs the function of seek control and track following. The seek function is initiated when a command is issued to read data from or write data to a target track on the disk. Once the transducer has been positioned sufficiently close to the target track by the seek function of the servo control system, the track following function of the control system centers and maintains the transducer on the target track until the desired data transfer is completed.
Typically, the transducer will oscillate about the centerline of the target track for a period of time following the transition of the servo control system from the seek mode to the track following mode. These off-track displacements, or post-seek oscillations (PSO), are due, at least in part, to mechanical vibrations generated by the components of the disk drive during the seek and/or tracking operation. In addition, while in the track following mode, adjustments to the position of the transducer with respect to the centerline of the target track are often required due to these same or similar mechanical vibrations. Such adjustments are required to correct drift in the position of the transducer relative to the target track. The precise control of the position of the transducer relative to a target track has become increasingly important as track densities (or tracks per radial inch—TPI) in disk drives have increased. More specifically, the number of tracks included on a disk, i.e., the greater the TPI, is proportional to higher data storage capability. However, the increased number of tracks means that there is a more stringent requirement that the transducer stay on track for both reading and writing purposes since the separation distance between adjacent tracks necessarily decreases as TPI increases. A measure of how far the transducer is off target is termed “Track Misregistration” (TMR). It can be measured in distance (e.g., microns) or as a percentage of track pitch. TMR is also referred to as off track or track following errors.
The actuator assembly generally includes one or more actuator arms interconnected to a component commonly known as an “E-block”. The E-block also includes an aperture for receiving a shaft and a pivot bearing assembly about which the actuator assembly freely rotates. Each actuator arm includes a load beam with a slider that secures a transducer as previously described. The actuator assembly also includes a yoke that supports a voice coil that is used to position the actuator arms. More specifically, the voice coil is a coil or wire that is selectively supplied current thereby altering its magnetic field. This dynamic field of the voice coil interacts with a static magnetic field of a permanent magnet positioned adjacent to the voice coil. As the two magnetic fields are attracted/repelled, the arm of the actuator assembly will transition over the disk. The magnetic force of the voice coil must be reacted by the shaft and pivot bearing of the actuator assembly. The stiffness of the shaft/bearing directly effects the position of the actuator head and TMR.
The negative effects of post seek oscillations and TMR are most easily described by a brief discussion of track pitch. The distance between two concentric tracks of a disk is known as track pitch, which decreases as TPI increases. For example, a disk with 100,000 TPI has generally a track pitch budget of 0.25 microns (approximately 10 millionths of an inch), wherein a disk with a 150,000 TPI has a track pitch of about 0.17 microns (approximately 7 millionths of an inch). As described above, each vibrating component of a disk drive has a budget that contributes to the maximum allowable TMR that are correctable by the servo control system. That is, vibrational induced oscillations of the transducer must be maintained at or below a level where the servo controller can effectively counteract the movement and control the position of the transducer. This level is predetermined in the design of a disk drive. Returning now to the above example in which TPI is increased from 100,000 to 150,000, and the same servo controller is used in each instance, vibrations generated by the disk drive components increase as a percentage of the total budget. Therefore, it is desirable to implement means of reducing vibrations such as by stiffening the actuator assembly and/or reducing mass or rotating inertia to effectively shift the frequency of the system mode.
As alluded to above, it is often desirable to stiffen the pivot bearing to counteract external loads from the voice coil. The E-block of the actuator assembly is often rotatably interconnected via a plurality of ball bearings to the stationary shaft. The stationary shaft includes a flange at one end for interconnection to the base plate of a disk drive housing. An upper ball bearing assembly and a lower ball bearing assembly are positioned around the external diameter of the shaft. Each ball bearing assembly is comprised of an inner race, which is interconnected to the stationary shaft, and an outer race, which is interconnected to the E-block. The inner and outer races are separated by a plurality of ball bearings. This type of assembly is generally known as a “sleeveless cartridge”, other pivot assemblies of the prior art include a cylindrical sleeve positioned exterior to the ball bearings.
Typical ball bearing assemblies are fabricated with the ball bearings evenly spaced about the inner race of the ball bearing assembly. Uniform distribution of balls results in a uniform radial stiffness of the ball bearing assembly, wherein “radial” refers to a direction perpendicular to the axis of the shaft. Traditional ball bearing placement is ideal for applications where the item, i.e. the shaft, is spinning a full 360 degrees. That is, ball bearing assembly employs axially symmetric inner and outer races that are shaped so that a radial load passes through the ball bearing. Most ball bearing assembly designs also support modest axial loads. Ball bearing assemblies also include a cage that is responsible for maintaining ball bearing spacing. Without the cage, the ball bearings would collide and lead to bearing assembly seizure and possibly bearing disassembly. Cages are often comprised of two cage halves made of thin pieces of metal that include indentations for receiving and spacing individual ball bearings. Once the balls are located within the indentations of the cage halves, the cage halves are secured such that the ball bearings remain spaced but are free to rotate.
Actuator assemblies of disk drives only traverse a range of about 20-30 degrees during normal read/write operations. During such rotation, the primary forces acting on the actuator shaft originate from the voice coil and act generally perpendicular, i.e. “lateral”, to the axis of the shaft. As used herein, “lateral” generally refers to a direction parallel to the plane of the rotating disk and perpendicular to the axis of the pivot bearing and perpendicular to the longitudinal axis of the actuator assembly. The longitudinal axis of the actuator assembly is defined as a line extending from the voice coil through the center of the pivot bearing to the actuator transducer. Other forces acting on the shaft of the actuator assembly are present, but are small compared to forces acting in the lateral direction. In order to react these lateral and non-lateral forces, it is desirable to stiffen the actuator pivot assembly, which has been done in the past by increasing the preload of the ball bearing assembly. The load of the ball bearing assembly can be generally viewed as tightening the inner race—ball bearing—outer race interconnection, i.e., squeezing the ball bearings. By increasing the preload, the stiffness provided by the ball bearing assembly increases. However, one detrimental effect of increasing preload is that friction of the ball bearing assembly increases which relates to a higher torque load needed to transition the actuator to a desired location on the disk. The torque load is directly proportional to the amount of energy required to operate the disk drive and thus battery life. In addition, by increasing friction in a system, more heat is generated, which is also detrimental to the operation of a disk drive. Higher friction is generally a result of higher pressures or stress levels in the bearings. These higher stress levels lead to a shortened bearing cartridge life. Finally, friction is a non-linear force comprised of static and dynamic components thereby making it difficult to address with the servo control. More specifically, when repositioning a stationary transducer head, more torque is needed to overcome the static friction. This rapid change in loading in magnitude and direction may lead to track overshoots, for example.